Field of the Invention
The present invention relates to a dental curable composition to be suitably used as a dental material that can substitute part or all of a natural tooth, or a resin material for dental cutting and machining, and a resin material for dental cutting and machining, a resin artificial tooth and a composite resin artificial tooth produced by the dental curable composition.
Description of the Related Art
A resin or a composite obtained by mixing a resin and an inorganic filler has been conventionally used with being molded into an artificial tooth in one treatment in the field of dentistry. In formation of the artificial tooth, the material of the artificial tooth has been filled in a mold and cured, but strain has been generated in curing to cause cracks and chipping to be generated.
In recent years, a dental CAD/CAM system has become widely used, and has enabled cutting and machining precisely so as to allow a prosthetic appliance to be produced. A resin block and a composite resin block, also having flexibility as a material to be cut and machined, are easy in occlusal adjustment and polishing operation at a restoration laboratory or chair side, and hardly damage an opposing tooth due to abrading of themselves, as compared with a ceramics block. Such a resin block and a composite resin block have been filled in a mold and cured as in the artificial tooth, however, strain has been generated in curing to cause cracks and chipping to be generated.
Japanese Patent Laid-Open No. 10-323353 discloses a dental resin material including a methacrylate or acrylate monomer and a thermal polymerization initiator. The (meth)acrylate monomer included in this dental resin material, however, has a high heat-curing reaction rate, therefore, for example, curing is rapidly initiated near a surface of a molded product, which is in contact with a mold, and volume shrinkage locally occurs due to polymerization of the monomer, and thus there is the following problem: strain is generated in a molded product to thereby cause cracks and chipping to occur and a uniform molded product cannot be obtained.
With respect to the resin block and the composite resin block, there has been a demand for a molding technique for producing a material for machining, having a large shape. A large molded product is difficult to produce as a uniform molded product with no incorporation of gas bubbles, the absence of strain, and no cracks and chips. In particular, methyl methacrylate is low in boiling point and high in polymerizability, therefore easily causes generation of strain and incorporation of gas bubbles due to foaming, and has also many problems in terms of productivity, for example, is required to be polymerized over a long time in order to be uniformly polymerized and cured.
Japanese Patent Laid-Open No. 2012-214398 discloses the following: polyethylene glycol dimethacrylate having a specific molecular weight can be contained in a specific amount to thereby provide a molded product without generation of cracks and chipping. A resin molded product uniformly thermally cured, however, cannot be obtained, and the following problem is caused: strain is still generated in a molded product to thereby cause cracks and chipping to be generated.
Japanese Patent Laid-Open No. 2011-037726 describes the invention of a bactericidal sealer for filling a root canal, in which terpinen-4-ol is compounded in order to impart the bactericidal effect.
National Publication of International Patent Application No. 2009-541375 and National Publication of International Patent Application No. 2007-526270 describe a dental material such as a material for an artificial tooth, in which terpinene is compounded as a stabilizer.
While a resin material for dental cutting and machining (resin block), having a large shape, for use in a temporary prosthetic appliance or a denture base is produced by admixing a powder material mainly including polymethyl methacrylate with a liquid material mainly including methyl methacrylate, and then filling the resulting admixture into a mold for molding and processing with pressure and heating, such an admixture has a high thermal conductivity and therefore a portion thereof, being present in the vicinity of the mold, starts to be rapidly polymerized. Therefore, the following problem is caused: strain is generated between the vicinity of the mold and the inside of the mold to cause cracks and chipping to be generated. In addition, since methyl methacrylate is a polymerizable monomer having a low boiling point, there is also the following problem: foaming occurs in the process of a rise in mold temperature to generate gas bubbles in a molded product.
A resin material for dental cutting and machining (composite resin block), for producing a temporary prosthetic appliance, a denture base or the like, has a filler such as a silica filler or an organic/inorganic composite filler compounded at a high density in order to exhibit aesthetic property and mechanical properties required for a crown prosthetic appliance. In this case, a material structure is adopted in which a polymerizable monomer is present around the filler with being polymerized for curing. While the composite resin block is produced by, for example, filling a paste-like dental curable composition including the filler and the polymerizable monomer into a mold for molding and processing with pressure and heating, the filler and the polymerizable monomer have a largely different thermal conductivity from each other, and therefore the following problem is caused: micro-strain is generated in the block to cause cracks and chipping. This is because the polymerizable monomer in which heat conducts quickly under pressure and heating for production of the block is rapidly polymerized. Moreover, in molding and processing in which the paste-like dental curable composition is pressurized and heated in the mold, while heat polymerization instantly progresses in the paste in the vicinity of the mold because of ease of conduction of heat, heat polymerization slowly progresses in the paste far from the mold and present near the inside of the mold, in which heat hardly conducts, as compared with the former case. Such ununiform progress of heat polymerization depending on the location of the paste causes the following problem: micro-strain is generated in the block to cause cracks and chipping. In addition, when the polymerizable monomer having a low boiling point, such as methyl methacrylate, is used, there is also the following problem: foaming occurs in the process of a rise in mold temperature to generate gas bubbles in a molded product.
The resin artificial tooth and the composite artificial tooth are made having a form similar to a natural tooth by stacking respective layers for polymerization and curing by a compression molding method in which raw materials for a paste-like admixture are filled in a mold and pressurized and heated, an injection molding method in which the admixture is injected as a raw material into a mold at a constant pressure, or the like. The difference in conduction of heat, however, is caused by, for example, ununiform component compositions and thicknesses of the layers stacked and hence the presence of a thinner portion and a thicker portion in the same layer, and furthermore the locational relationship (for example, the vicinity of the mold or the inside of the mold) from the mold in filling of the admixture into the mold. As a result, the rate of polymerization and curing is changed, and there is the following problem: micro/macro-strain is generated to cause defects such as partial shrinkage, cracking, clouding and chips in molding with pressure and heating. In addition, when the respective layers are sequentially stacked with polymerization and curing, there is also the following problem: adhesiveness between the layers is insufficient due to the influences of compatibility in pressure-welding of a raw material for providing a new layer with polymerization and curing, to the surface of the layer cured, and of the polymerization shrinkage stress in the polymerization and curing, and physical properties intended are not exhibited.
Furthermore, a monofunctional (meth)acrylate monomer to be mainly used as a component of the resin artificial tooth has a low boiling point and therefore is to be foamed by a rapid rise of temperature to cause gas bubbles to be easily incorporated in polymerization and curing. There are also many problems in terms of productivity, for example, polymerization and curing are required to be performed over a long period of time in order to achieve uniform polymerization and curing.
From the foregoing, an object of the present invention is to provide a dental curable composition in which, when a block-shaped or disc-shaped molded product usable as a resin material for dental cutting and machining is produced while mechanical properties such as hardness, bending strength and compressive strength as well as aesthetic property required for a temporary prosthetic appliance, a denture base and a crown prosthetic appliance are maintained, the dental curable composition can be molded and processed with pressure and heating while strain generated in the molded product is reduced, no cracks and chipping occur, and no gas bubbles are incorporated due to foaming.
Another object of the present invention is to provide a resin artificial tooth in which, during molding and processing with pressure and heating for replicating a form similar to a natural tooth while respective layers of the resin artificial tooth are stacked by polymerization and curing, uniform polymerization and curing are achieved with no influences of the component compositions of the respective layers, the thickness of a layer structure and the like, strain generated in the resin artificial tooth is reduced to thereby inhibit defects such as local shrinkage, cracking, clouding and chips from being generated, molding and processing under pressure and heating conditions that cause no gas bubbles to be incorporated due to foaming can be conducted, and the respective layers firmly adhere not to adversely affect material properties.